Since there have been a few threads already asking this question, let me state once more clearly what is also on the website:

This browser is a close twin to pre-Servo Firefox in how it operates.

So in general:"Will basilisk support/have {feature X}?" - "Does Mozilla Firefox from early 2017 support/have {feature x}? If so, then yes. If not, then no."

It will look and operate much like a pre-Servo version of Mozilla Firefox, as Basilisk's application code is very close to it.

Obvious exceptions are a few things that require specific commercial partnerships like Google Safebrowsing, and things that are Mozilla infrastructure specific like healthreport, crash reporting and telemetry, as well as bundled third-party fluff like Pocket, Firefox screenshots and similar.

Improving Mozilla code: You know you're on the right track with code changes when you spend the majority of your time deleting code.